The present invention generally relates to apparatus for filtering and screening particulate matter in downhole wells, and, more particularly, to an improved method of manufacturing a prepacked well liner for filtering unconsolidated material out of inflowing well fluid such as water, oil or gas.
In the course of completing an oil and/or gas well, it is common practice to run a string of casing into the well bore and then to run the production tubing inside the casing. At the well site, the casing is perforated across one or more production zones to allow production fluids to enter the casing bore. During production of the formation fluid, formation sand is also swept into the flow path. The formation sand is relatively fine sand that erodes production components in the flow path. In some well completions, the well bore is uncased, and an open face is established across the oil or gas bearing zone. Such open bore hole arrangements are utilized, for example, in water wells, test wells, and horizontal well completions.
It is generally known that the production of fluids from an underground producing formation is often reduced or is completely chocked off by the movement of sand and/or other finely divided solid materials into the well bore. Sand and finely divided solids are also troublesome when they are entrained in the producing fluids because of abrasion of pump parts and other equipment.
It is a general practice to employ strainers or screening devices to overcome the above noted problems. A wide variety of well filter devices have been devised to prevent the movement of sand and other materials into the producing zone of wells. Such devices usually consist of a perforated metal mandrel generally employed in association with a fine mesh screen. The perforated metal mandrel and fine mesh screen are installed in the flow path between the production tubing and the perforated casing (cased) or the open well bore face (uncased). In a procedure called gravel packing, the annulus around the screen is packed with a relatively coarse sand or gravel which acts as a filter to reduce the amount of fine formation sand reaching the screen. The gravel and sand, which is often called a proppant, is pumped down the well in a slurry of water or gel. In well installations in which the screen is suspended in an uncased open bore, the gravel pack further supports the surrounding unconsolidated formation.
A problem, which arises during initial production following the gravel packing operation, is that fine sand may be carried through the gravel pack before the gravel pack stabilizes. It is not unusual to produce a substantial amount of fine sands before the gravel pack finely consolidates and yields clean production. After gravel packing and during the early stages of well production, these fines tend to migrate through the gravel packing and screen and lodge within the inner annulus between the wire wrap and the perforated mandrel. In some instances, this can cause severe erosion of the screen and ultimate failure of the screen to reduce sand invasion. In other situations, the sand fines may include plugging materials which can completely plug the mandrel flow passages and terminate well production shortly after completion. In deep wells, when the screen becomes plugged and the pressure in the production tubing is reduced, the formation pressure can collapse the screen and production tubing. Moreover, when a substantial amount of sand has been lost from the surrounding formation, the formation may collapse with a resultant damage to the well casing or liner and consequent reduction or termination of production.
A further problem with gravel packing is that gravel packed liners can deteriorate over time and can be difficult to replace because removal from the well bore can cause the metal screen to become entangled or broken during removal from the well. Gravel packing can also create a non-uniform filter in high angle and horizontal wells due to the difficulty in the proper placement of the sand or gravel with current packing techniques. Additionally, the gravel packing procedure is both expensive and complicated.
One attempt to overcome the foregoing problems of gravel packing has been to interpose a prepack of bonded proppant between the perforated mandrel and the perforated casing or the open well bore face. The mesh wire screen is replaced by a permeable filter body of granular material, for instance gravel, which is cemented together by a binding agent which is insoluble by water or other liquid to be filtered and which is permanently adhered to the perforated pipe. See, for example, U.S. Pat. No. 2,843,209 issued to W. Degen and U.S. Pat. No. 3,361,203 issued to R. Rensvold which are incorporated herein by reference. The binding agent is typically an artificial resin such as a thermoplastic or thermosetting resin such as phenol formaldehyde, urea formaldehyde or melamine formaldehyde. The grain size of the gravel or sand is adapted to the soil conditions surrounding the filter which may be uniform or it may comprise several layers containing different binders and proppants.
Unfortunately, the process lacks control in the ability to provide uniform interstitial spaces between proppant grains. During the curing process the resin remains free flowing thereby allowing the resin coating to flow into the interstitial regions between proppant grains. The resin in the proppant interstices results in random plugging of the pore spaces during the curing process. This has resulted in such diminished porosity and permeability of the well filter that such well liner has not continued in general use in the well drilling industry.
An attempt to overcome the problems of resin plugging of a prepacked liner is disclosed in U.S. Pat. No. 3,683,056 issued to Brandt et al. The resin bonded particles are cured in a mixture of hot wax to cause the bonding agent to polymerize and consolidate the particle pack while allowing the wax to cool in and around the particle pack and in the interstitial regions between contiguous particles. Once the prepacked liner is installed in the well, the formation heat causes the wax to melt and be removed from the prepack thereby providing for a porous and permeable matrix of resin bonded particles. Unfortunately, the subject method is not suitable where formation temperatures are insufficient to melt the wax disposed in the interstitial regions of the matrix. Furthermore, the wax procedure is both expensive and complicated involving complicated machinery to pump and mix the molten wax bath.